In certain extracorporeal blood treatments, a coagulation prophylaxis takes place in order to prevent coagulation of the extracorporeally conducted blood. In a known method that serves this purpose, a coagulation tendency of the blood flowing extracorporeally is reduced by initially adding citrate to the blood. By adding the citrate, calcium is increasingly permeably transferred through the membrane of the utilized filter. This and also using solutions during the blood treatment that do not comprise calcium, lead to a calcium loss from the blood. After having treated the blood, calcium in the form of a calcium-containing solution is added to the blood before it is returned into the body of the patient. Methods and apparatuses for determining an amount or concentration of calcium necessary for achieving this effect are known from practice.